Copper fields: Quantum criticality in high-temperature cuprate superconductors

June 19, 2012 by Stuart Mason Dambrot, feature

Doping evolution of magnetic field-temperature phase diagrams of La2-xCexCuO4. (A-D) The magnetic field dependence of the evolution of superconducting (yellow), Fermi liquid (blue), and non-Fermi liquid (red, white) ground states of the electron-doped cuprate system La2-xCexCuO4 is shown for several electron doping levels (x). Image Copyright © PNAS, doi: 10.1073/pnas.1120273109
( -- Superconductivity is a complex phenomenon that is considerably more intricate than many casual observers realize. This caveat applies equally to the subset of this research known as high-temperature superconductivity – which, it should be noted, is described as such only in relation to the near absolute zero temperature range at which conventional superconductors are found, and furthermore is not to be confused with the loftier goal of room-temperature superconductivity. That said, certain aspects of electronic properties in high-temperature copper oxide, or cuprate, superconductors imply that the absence of conventional metallic Fermi liquid behavior – the standard model of electrons in metals – and the presence of unconventional superconductivity are closely related. While such a partnership often occurs proximate to what is known as a quantum critical point (a special class of continuous phase transition that takes place at the absolute zero of temperature in a material where the phase transition temperature has been driven to zero by the application of a pressure, field or through doping), the role of quantum criticality in the cuprates has remained elusive. Recently, however, researchers at the Center for Nanophysics and Advanced Materials and Department of Physics, University of Maryland, have studied the anomalous properties of the cuprate material La2-xCexCuO4, or LCCO, concluding that quantum criticality plays a significant role in shaping the anomalous properties of these superconductive materials.

The research team of Asst. Prof. Johnpierre Paglione faced significant challenges in designing and implementing an experiment to determine the relative importance of quantum criticality in the cuprates. “The relevance of quantum criticality to shaping the properties of the cuprates is a pivotal question that remains unsolved,” Paglione tells “There are several ‘phases’ of these materials, including insulating, anomalous metallic, superconducting and normal metallic ground states – and understanding how these phases arise, coexist, and interact will allow for understanding the underlying physics that gives rise to them.” This is, he adds, the key impetus motivating researchers to understand the phase diagram of the cuprates.

“One of the main challenges in doing so,” Paglione continues, “lies in the fact that there are few actual compounds that can be tuned through the full phase diagram – that is, from parent compound Mott insulator, through the anomalous non-Fermi liquid metallic/superconducting phase, to over-doped ‘normal’ metal. Well-known compounds like yttrium barium copper oxide (YBCO) have a limited range of chemical tuning – by way of oxygen doping or rare earth substitution – which make it difficult to study their properties through all three of these regions of the phase diagram, resulting in the necessity to patch together results from different systems to obtain what is widely held to be the generic cuprate phase diagram.” Luckily, Paglione points out, there are some systems, such as the "hole-doped" (La,Sr)2CuO4 and "electron-doped" (La,Ce)2CuO4 that allow for a wide range of substitution – and ongoing work has made use of these to answer many questions.

“Another challenge,” Paglione explains, “lies in understanding the details and importance of some of the anomalous normal state properties, that is, above the temperature where superconductivity exists. In particular, most studies to date have focused on the hole-doped cuprates because their superconducting phases exhibit the highest transition temperatures, but these systems all harbor the infamous ‘pseudogap’ phase (a state where the Fermi surface of a material possesses a partial gap) on the underdoped side of the superconducting phase. Much effort has been spent on understanding whether this phase is a ‘friend or foe’ of superconductivity.”

Another issue was that cuprate superconductivity is an amazing phase, having the highest transition temperatures and critical magnetic fields of any known material – but for the purposes of studying the underlying ground state that gives rise to Cooper pairing this is actually a key difficulty. ”Specifically,” Paglione notes, “to study the true ground state properties at the lowest energy scales – those approaching absolute zero – superconductivity must be somehow suppressed. Such high temperatures and fields make it extremely difficult to do high-sensitivity experiments in a practical manner.”

Addressing these issues required no small amount of ingenuity, as well as the particular LCCO compound used in the study. “Firstly,” notes Paglione, “this compound is unique in that it can be synthesized with a wide range of (Ce) concentrations that span across the entire phase diagram, from insulating (x=0) to overdoped metallic (x>0.175). Secondly, this is an electron-doped system, in which the anomalous pseudogap phase is absent. Therefore comparisons of its anomalous properties to those of its hole-doped counterparts with the pseudogap present has allowed us to identify traits common to both, and therefore not associated with the pseudogap phase.”

This includes, most importantly, the linear-in-temperature normal state resistivity that surrounds the superconducting phase. “We can now associate this anomalous scattering with the presence of well-characterized magnetic fluctuations that are almost magnetically ordered, giving strong reinforcement to the well-known hypothesis of quantum critical fluctuations dominating the properties of the normal state,” Paglione explains.

The lower transition temperatures and critical fields of this system allowed the team to probe the ground state in their lab using high but practically attainable extremes of temperature (0.020 Kelvin) and magnetic field (17 Tesla). “With this, we discovered a striking quantum critical scaling of the physical properties with the ratio of temperature to field, which is not seen in a normal metal due to an upper energy scale of the electron system – the Fermi energy – that dominates all characteristics of the metal, such that the normally static factor of temperature is now dynamic. This is a telltale sign of quantum criticality.”

In addition, Paglione points out, there are other innovations that might be developed and applied to the current experimental design. “Due to the crystallographic nature of the LCCO system, it is only stable in thin-film form and therefore single-crystalline samples cannot be synthesized easily.” This is a drawback, since further study of the thermodynamic properties of the system – such as heat capacity and magnetization, which require more sample mass to detect – in the quantum critical regime would allow further insight into its physical description and origin. “Higher magnetic fields and even lower temperatures would allow us to extend the range of applicability of the anomalous properties and therefore further elucidate the extent to which standard model solid state physics fails to explain these materials,” says Paglione.

Relatedly, the team has already defined the next steps to be applied to their research. “Currently we’re most interested in observing the same phenomena in other cuprate materials, allowing for more general conclusions to be made. In particular,” Paglione illustrates, “the similarities of the overdoped side of the phase diagram between hole- and electron-doped cuprates are striking, and they carry a strong potential statement about what governs the demise of superconductivity with increasing doping but require further verification in different compounds.”

One particularly interesting outcome of the study is how the team’s findings impact the development of using the selective response of spin fluctuations and superconductivity to magnetic fields and charge doping to segregate the resulting two distinct signatures of criticality. “The observation of two distinct signatures of quantum criticality is an interesting find. We expected to see only one signature of critical magnetic fluctuations, as given by one set of critical exponents, since an incipient magnetic order and its strong quantum fluctuations are what most people believe lies ‘underneath the dome’ and drives the Cooper pairing. The observation of a second flavor of quantum fluctuations, which we conclude arises from the demise of the superconducting transition temperature to absolute zero, suggests that fluctuations of this separate type of order – that is, superconducting, not magnetic – can also cause anomalous physical properties to arise. These may in fact be the cause of most properties associated with the overdoped side of the phase diagram, including the suppression of superconductivity itself and the strong tuning of the eventual normal metal Fermi liquid ground state.”

Paglione points out that a theory exists for this type of superconducting fluctuation, but a calculation of the expected transport properties has not been done. However, he adds that they are working with Prof. Galitski at the University of Maryland to do just this.

Finally, Paglione explains, there are other research areas and applications that might benefit from their findings. “The study of is a wide field of research that overlaps with superconductivity in several areas, including the cuprates but also in heavy-fermion systems and the more recent discovery of superconductivity in iron-based materials. Our results have implications for all of these areas, where a strong interplay of superconductivity and magnetism results in an easily tunable of the sort found in the cuprates. Beyond that,” he concludes, “a deeper understanding of high-temperature superconductivity in cuprates and iron-pnictides has the potential to allow for crystallographic engineering of new compounds that could harbor even higher superconducting transition temperatures useful for room temperature power, electronic and communications applications.”

Explore further: Swimming upstream: Flux flow reverses for lattice bosons in a magnetic field

More information: Quantum critical scaling at the edge of Fermi liquid stability in a cuprate superconductor, PNAS, Published online before print May 9, 2012, doi: 10.1073/pnas.1120273109

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1 / 5 (7) Jun 19, 2012
It's not so difficult to imagine it. The superconductivity arises, when the particles of vacuum shake the electrons stronger, than the thermal vibrations of the atom lattice. This usually restricts the superconductivity to the low temperatures. But it's possible to compress the electrons in such a way, their repulsive forces will overlap and compensate mutually. It's similar mechanism to liquefaction of material under high pressures (during explosions, the bazooka weapon is penetrating with this principle). The electrons are too tiny, they cannot be compressed between atoms, but they can be attracted to positively charged places of atom lattice (so called the holes) like the hens to their feeder. The elevated concentration of electrons along the hole stripes forms the stripes of chaotic electron fluid within atom lattice, which transport the highly compressed chaotic electrons fluid in Mexican waves, i.e. the superconducting paths within material.
1 / 5 (8) Jun 19, 2012
Now we may think, what will happen, if we change the concentration of positively charged atoms (holes) within atom lattice. If we increase it, we increase the number of places, where the electrons can be compressed, but the general level of their compression will be lower. At the high level of doping the material will get the metallic conductivity, i.e. low resistance, but the superconductivity will not start at all even at low temperature.

When we decrease the level of doping, then the opposite effect will occur. The electrons will be indeed compressed more, but their islands will become more and more isolated. We'll get so-called "pseudogap" state: the material will be full of superconductive islands around each doping atom, but because these islands will not form the continuous phase, this material will not become superconductive in the bulk anyway and all these doping atoms will be wasted.
1 / 5 (8) Jun 19, 2012
The general memo is, the main trick in preparation of really high-temperature superconductors is in formation of continuous hole stripes, i.e. lines of oxygen doped atoms, which are as sparsely separated, as possible (the semiempirical Rosser's equation accounts into it). These two requirements are indeed contradictory for most of common crystalline materials: the more sparse doping level we provide, the lower probability of continuous stripes of dopant atoms we get. It's solely geometrical problem, independent on the nature of material.
1.1 / 5 (8) Jun 19, 2012
Now you can try to answer, whether the "pseudogap phase is a friend or foe of superconductivity" for yourself. IMO the physicists would do better, if they would try to replicate the findings of Joe Eck and J.F.Prins, who didn't babble about trivialities, but who really succeeded with preparation of room temperature superconductor. We are losing lotta money because of ignorant approach of mainstream physics, which is (not quite accidentally) similar to situation in cold fusion research. The community of physicists doesn't follow the most successful researchers, but the average ones, which is indeed bad.
1 / 5 (2) Jun 20, 2012
Downvoted with verkle, sirchick, atomsk and BoxPopuli. What a coincidence.
1 / 5 (4) Jun 20, 2012
Downvoted with verkle, sirchick, atomsk and BoxPopuli. What a coincidence.

Why does anyone care about ratings, just focus on the strength of posting with the power of ones intellect and dont let your emotions get harangued at all and please dont comment on that concern, there will be LESS noise and more signal, sensible ?
5 / 5 (2) Jun 21, 2012
Downvoted with verkle, sirchick, atomsk and BoxPopuli. What a coincidence.

Me too ;)
1 / 5 (2) Jun 21, 2012
I dunno, why my posts are getting so downvoted. I consider them quite insightful and clever. For example, the above mechanism explains, why elements like the sodium can never get superconductive (even at the high pressures), whereas the elements like the niobium yes. The sodium has many conductive electrons around itself, but it lack the elongated orbitals, which would connect the atoms together and compressed the electrons between them. As the result, the sodium is soft metal full of electrons, but it lack the superconductivity.

Instead of it, the elements like the niobium have d- orbitals, which extend the diameter of other orbital and form the rigid cage, inside of cells of which the electrons from another orbitals may may be squeezed. This is general mechanism of superconductivity so-called Type I superconductors.
1 / 5 (2) Jun 21, 2012
It's evident, the possibility of electron compression between orbitals of atoms of single type are quite limited. A way better possibility is provided with lattice of materials, which is able to form robust structures from many atoms at the same moment. These materials form the Type II superconductor, i.e. the high-temperature superconductors in common sense. The level of electron compression may be much higher there and the number of electrons squeezed as well. It just requires to separate hole stripes sufficiently with inert oxide layers, so that the repulsive forces of electrons are balanced with attractive forces of sufficient number of atoms. For example, if we would shot the straight lines of oxygen atoms into diamond lattice with method of ion implantation, we could achieve the superconductive stripes of extremely high temperature of superconductive transition.
1 / 5 (2) Jun 21, 2012
Before some time prof. J.F.Prins experimented with hole doping of artificial diamond layers. When he implanted the oxygen atoms beneath the surface of diamond a strange phenomena occurred in vacuum. The electrons were attracted to the surface of diamond above the places, where oxygen atoms resided and the free electrons formed a superconductive phase there. It was first artificial superconductor, working at the room temperature, but this finding was widely ignored by mainstream physics, although it was presented in NewScientist journal and many others. Apparently mainstream physics has quite different priorities, than to develop solution, which could be useful for the rest of civilization. Instead of it, it develops the solutions, which are useful for scientists itself, i.e. which enable to pile another and another publications and to ask money for neverending researc
not rated yet Jun 21, 2012
...this finding was widely ignored by mainstream physics...

To be thorough and impartial, you should have noted that after receiving the peer response you describe, Prins acknowledged that he still had to show that the state can expel magnetic fields to conclusively prove that it is superconducting - and I am not aware of his ever doing this. That said, however, it is also the case that he had recently retired and no longer had the facilities to perform such an experiment - and he offered to fly his samples to another lab but found no takers.
not rated yet Jul 19, 2012
TkClick, how does that theory explain that Lithium become superconducting at 20 K with 48 GPa pressure, and even without pressure, at 0.4 mK, but "Sodium can never get superconductive"?
1 / 5 (1) Jul 19, 2012
IMO the sodium atoms are rather bulky and when they're compressed, then their free electrons find free spaces between atom nuclei, where they evade further compression. They become unmovable during this and as the result the metallic sodium becomes transparent electride at high pressures. But the lithium atoms are smaller and their electrons have not so much of place to hide between atoms during their compression. Therefore it's possible to achieve the superconducting state at the case of lithium and of course hydrogen too. The relativistic effects may account to this behaviour too, because the electrons revolving large atoms are forced to move with higher speed and they gain the relativist mass because of it. This brings them closer to the atom nuclei, where they're more difficult to compress.
1 / 5 (4) Jul 19, 2012
Stupid to worry about votes, we are not children, focus as adults on the content of postings please. I have got useless notifications about votes - waste of time !

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